1. Field of the Invention
The field of the invention is that of hydrophonic antennas, more particularly those designed for the detection of targets.
2. Description of the Prior Art
It is well known that the directivity of an linear antenna consisting of omnidirectional hydrophones is omnidirectional in the plane perpendicular to the axis of the antenna. In particular, it is not possible to determine whether a target is located to the right or to the left of the antenna.
However, it is possible to obtain a directivity by designing an antenna with two orthogonal dipoles. As shown in FIG. 2, if D is the distance between the two sensors 22, 23 and 25, 26 of each dipole 21 and 24, the function of directivity is ##EQU1## for one of the dipoles and ##EQU2## for the other dipole, .theta. representing the direction. If the wavelength is not far greater than D, then we find the direction by tg.theta..
It is advantageous to make an antenna comprising at least one directional module working on this principle.
Now, should it be desired to be able to obtain a horizontal and/or vertical directivity, for example, it is necessary to have several aligned hydrophones with combined signals. In this case, we come up against the difficulty of the adjusting of the directional modules with respect to one another.
There is a seismic exploration system, described in the U.S. Pat. No. 4,789,968 which can be towed by a ship and enables the selective detection of waves coming from a particular direction which is such that the angle of incidence and the amplitude of the detected waves can be determined. This device has hydrophones formed by orthogonal dipoles consisting of semi-cylindrical elements made of piezo-electrical crystal, the hydrophones being arranged along a cylinder. A system such as this can be used to determine the direction of the compression waves and the shearing waves detected by measuring their speed through a field to be analyzed. This device is essentially designed to make probes in a field, but it is not suited to the detection of sound waves coming from a submerged target.